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<div class="titlepage"><div><div><h4 class="title">
<a name="phoenix.modules.core.references"></a><a class="link" href="references.html" title="References">References</a>
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<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">phoenix</span><span class="special">/</span><span class="identifier">core</span><span class="special">/</span><span class="identifier">reference</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
</pre>
<p>
          Values are immutable constants. Attempting to modify a value will result
          in a compile time error. When we want the function to modify the parameter,
          we use a reference instead. For instance, imagine a lazy function <code class="computeroutput"><span class="identifier">add_assign</span></code>:
        </p>
<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">add_assign</span><span class="special">(</span><span class="identifier">T</span><span class="special">&amp;</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">T</span> <span class="identifier">y</span><span class="special">)</span> <span class="special">{</span> <span class="identifier">x</span> <span class="special">+=</span> <span class="identifier">y</span><span class="special">;</span> <span class="special">}</span> <span class="comment">// pseudo code</span>
</pre>
<p>
          Here, we want the first function argument, x, to be mutable. Obviously,
          we cannot write:
        </p>
<pre class="programlisting"><span class="identifier">add_assign</span><span class="special">(</span><span class="number">1</span><span class="special">,</span> <span class="number">2</span><span class="special">)</span> <span class="comment">// error first argument is immutable</span>
</pre>
<p>
          In C++, we can pass in a reference to a variable as the first argument
          in our example above. Yet, by default, the library forces arguments passed
          to partially applied functions to be immutable values (see <a class="link" href="values.html" title="Values">Values</a>).
          To achieve our intent, we use:
        </p>
<pre class="programlisting"><span class="identifier">expression</span><span class="special">::</span><span class="identifier">reference</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">type</span>
</pre>
<p>
          This is similar to <code class="computeroutput"><span class="identifier">expression</span><span class="special">::</span><span class="identifier">value</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">type</span></code>
          before but instead holds a reference to a variable.
        </p>
<p>
          We normally don't instantiate <code class="computeroutput"><span class="identifier">expression</span><span class="special">::</span><span class="identifier">reference</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">type</span></code>
          objects directly. Instead we use:
        </p>
<pre class="programlisting"><span class="identifier">ref</span><span class="special">(</span><span class="identifier">v</span><span class="special">)</span>
</pre>
<p>
          For example (where <code class="computeroutput"><span class="identifier">i</span></code> is
          an <code class="computeroutput"><span class="keyword">int</span></code> variable):
        </p>
<pre class="programlisting"><span class="identifier">add_assign</span><span class="special">(</span><span class="identifier">ref</span><span class="special">(</span><span class="identifier">i</span><span class="special">),</span> <span class="number">2</span><span class="special">)</span>
</pre>
<h6>
<a name="phoenix.modules.core.references.h0"></a>
          <span class="phrase"><a name="phoenix.modules.core.references.evaluating_a_reference"></a></span><a class="link" href="references.html#phoenix.modules.core.references.evaluating_a_reference">Evaluating
          a Reference</a>
        </h6>
<p>
          References are actors. Hence, references can be evaluated. Such invocation
          gives the reference's identity. Example:
        </p>
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">3</span><span class="special">;</span>
<span class="keyword">char</span> <span class="keyword">const</span><span class="special">*</span> <span class="identifier">s</span> <span class="special">=</span> <span class="string">"Hello World"</span><span class="special">;</span>
<span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">ref</span><span class="special">(</span><span class="identifier">i</span><span class="special">)()</span> <span class="special">&lt;&lt;</span> <span class="identifier">ref</span><span class="special">(</span><span class="identifier">s</span><span class="special">)();</span>
</pre>
<p>
          prints out "3 Hello World"
        </p>
<h6>
<a name="phoenix.modules.core.references.h1"></a>
          <span class="phrase"><a name="phoenix.modules.core.references.constant_references"></a></span><a class="link" href="references.html#phoenix.modules.core.references.constant_references">Constant
          References</a>
        </h6>
<p>
          Another free function
        </p>
<pre class="programlisting"><span class="identifier">cref</span><span class="special">(</span><span class="identifier">cv</span><span class="special">)</span>
</pre>
<p>
          may also be used. <code class="computeroutput"><span class="identifier">cref</span><span class="special">(</span><span class="identifier">cv</span><span class="special">)</span></code>
          creates an <code class="computeroutput"><span class="identifier">expression</span><span class="special">::</span><span class="identifier">reference</span><span class="special">&lt;</span><span class="identifier">T</span> <span class="keyword">const</span><span class="special">&gt;::</span><span class="identifier">type</span></code>
          object. This is similar to <code class="computeroutput"><span class="identifier">expression</span><span class="special">::</span><span class="identifier">value</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">type</span></code>
          but when the data to be passed as argument to a function is heavy and expensive
          to copy by value, the <code class="computeroutput"><span class="identifier">cref</span><span class="special">(</span><span class="identifier">cv</span><span class="special">)</span></code>
          offers a lighter alternative.
        </p>
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<td align="right"><div class="copyright-footer">Copyright © 2002-2005, 2010, 2014, 2015 Joel de Guzman, Dan Marsden, Thomas
      Heller, John Fletcher<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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